Arcjet propulsion plants have an anode and a cathode. The anode forms part of a housing that also forms an expansion nozzle. The cathode is arranged and electrically insulated inside the housing. An electric discharge or arc is generated between the cathode and anode while flowing a propellant through the space between the tip of the cathode and the anode. The electrically insulated cathode reaches into a first recess of the housing functioning as a plenum chamber. The cathode tip is spaced by a small air gap upstream of the restrictor or expansion nozzle. The plenum is injected into the combustion chamber.
Such arcjets are also referred to as thermal arcjets. An article entitled "Cathode Erosion Tests for 30 kW Arcjets" by W. D. Deininger, A. Chopra and K. D. Goodfellow, was published in July 1989 as paper A/AA 89-2264 at the Joint Propulsion Conference of the A/AA/ASME/SAE/AS in Monterey, Calif., Jul. 10 to 12, 1989. A thermal propulsion plant is also disclosed in German Patent Publication DE 3,931,733 A1 or in U.S. Pat. No. 3,759,734. The propellant used in such arcjets is normally either ammonia (NH.sub.3) or a gas mixture produced by thermal and/or catalytic decomposition of hydrazin (N.sub.2 H.sub.4), whereby the mixture includes ammonia, nitrogen (N.sub.2) and hydrogen (H.sub.2). The propellant which has a temperature within the range of 500.degree.to 600.degree. C. at the time of its entry into the plenum chamber, is heated by the arc discharge between the anode and the cathode, whereby the temperature is increased into a range of 10,000.degree.to 15,000.degree. C. prior to the gas leaving the expansion nozzle to produce the desired propulsion thrust.
Most of the kinetic energy of the arc that establishes itself upon ignition when propellant flows through the plant, is transferred to the fuel gas in the area of the nozzle neck or constrictor. The arc establishes itself on a path from the anode through the nozzle neck to the tip of the cathode which normally has a conical configuration. This path of the arc extends precisely along the central axis of the constrictor or nozzle neck.
Thermal arcjet propulsion plants using ammonia as propellant have been constructed for substantially all power classes. These plants using ammonia have the advantage of a relatively simple handling without any problems compared to plants using hydrazin. However, ammonia using plants have a smaller specific thrust than hydrazin using plants.